Re: [Matplotlib-users] Adding coordinates to a plot in text form

[Nils W. <nw...@ia...>]

John Hunter wrote:
> On 2/8/07, Nils Wagner <nw...@ia...> wrote:
>
>> Is there a way to add the coordinates in text form to each plus in the
>> attached figure ?
>> ERach plus in the plot is generated by
>> plot([data[-1].real],[data[-1].imag],'k+')
>>
>> For example the rightmost plus (in the upper right half plane) should
>> have a text (1.049+0.692j)
>> Can I use text for this purpose ?
>
> You can use text, but I suggest you use the brand-spanking-new
> annotate function, which is designed to annotate data points with
> text.  It has support for arrows, and offsets from the annotated point
> in a variety of coordinate systems.
>
> Here is an example that shows annotations in a variety of contexts
>
> """
> Some examples of how to annotate points in figures.  You specify an
> annotation point xy=(x,y) and a text point xytext=(x,y) for the
> annotated points and text location, respectively.  Optionally, you can
> specify the coordinate system of xy and xytext with one of the
> following strings for xycoords and textcoords (default is 'data')
>
>
>  'figure points'   : points from the lower left corner of the figure
>  'figure pixels'   : pixels from the lower left corner of the figure
>  'figure fraction' : 0,0 is lower left of figure and 1,1 is upper, right
>  'axes points'     : points from lower left corner of axes
>  'axes pixels'     : pixels from lower left corner of axes
>  'axes fraction'   : 0,1 is lower left of axes and 1,1 is upper right
>  'data'            : use the axes data coordinate system
>
> Optionally, you can specify arrow properties which draws and arrow
> from the text to the annotated point by giving a dictionary of arrow
> properties
>
> Valid keys are
>
>          width : the width of the arrow in points
>          frac  : the fraction of the arrow length occupied by the head
>          headwidth : the width of the base of the arrow head in points
>          shrink : move the tip and base some percent away from the
>                   annotated point and text
>          any key for matplotlib.patches.polygon  (eg facecolor)
>
> For physical coordinate systems (points or pixels) the origin is the
> (bottom, left) of the figure or axes.  If the value is negative,
> however, the origin is from the (right, top) of the figure or axes,
> analogous to negative indexing of sequences.
> """
>
>
> from pylab import figure, show, nx
> from matplotlib.patches import Ellipse
>
> if 1:
>    # if only one location is given, the text and xypoint being
>    # annotated are assumed to be the same
>    fig = figure()
>    ax = fig.add_subplot(111, autoscale_on=False, xlim=(-1,5),
> ylim=(-3,5))
>
>    t = nx.arange(0.0, 5.0, 0.01)
>    s = nx.cos(2*nx.pi*t)
>    line, = ax.plot(t, s, lw=3, color='purple')
>
>    ax.annotate('axes center', xy=(.5, .5),  xycoords='axes fraction',
>                horizontalalignment='center', verticalalignment='center')
>
>    ax.annotate('pixels', xy=(20, 20),  xycoords='figure pixels')
>
>    ax.annotate('points', xy=(100, 300),  xycoords='figure points')
>
>    ax.annotate('local max', xy=(3, 1),  xycoords='data',
>                xytext=(0.8, 0.95), textcoords='axes fraction',
>                arrowprops=dict(facecolor='black', shrink=0.05),
>                horizontalalignment='right', verticalalignment='top',
>                )
>
>    ax.annotate('a fractional title', xy=(.025, .975),
>                xycoords='figure fraction',
>                horizontalalignment='left', verticalalignment='top',
>                fontsize=20)
>
>    # use negative points or pixels to specify from right, top -10, 10
>    # is 10 points to the left of the right side of the axes and 10
>    # points above the bottom
>    ax.annotate('bottom right (points)', xy=(-10, 10),
>                xycoords='axes points',
>                horizontalalignment='right', verticalalignment='bottom',
>                fontsize=20)
>
>    fig.savefig('annotation_coords')
>
> if 1:
>    # you can specify the xypoint and the xytext in different
>    # positions and coordinate systems, and optionally turn on a
>    # connecting line and mark the point with a marker.  Annotations
>    # work on polar axes too.  In the example below, the xy point is
>    # in native coordinates (xycoords defaults to 'data').  For a
>    # polar axes, this is in (theta, radius) space.  The text in this
>    # example is placed in the fractional figure coordinate system.
>    # Text keyword args like horizontal and vertical alignment are
>    # respected
>    fig = figure()
>    ax = fig.add_subplot(111, polar=True)
>    r = nx.arange(0,1,0.001)
>    theta = 2*2*nx.pi*r
>    line, = ax.plot(theta, r, color='#ee8d18', lw=3)
>
>    ind = 800
>    thisr, thistheta = r[ind], theta[ind]
>    ax.plot([thistheta], [thisr], 'o')
>    ax.annotate('a polar annotation',
>                xy=(thistheta, thisr),  # theta, radius
>                xytext=(0.05, 0.05),    # fraction, fraction
>                textcoords='figure fraction',
>                arrowprops=dict(facecolor='black', shrink=0.05),
>                horizontalalignment='left',
>                verticalalignment='bottom',
>                )
>    #fig.savefig('annotation_polar')
>
> if 1:
>    # You can also use polar notation on a cartesian axes.  Here the
>    # native coordinate system ('data') is cartesian, so you need to
>    # specify the xycoords and textcoords as 'polar' if you want to
>    # use (theta, radius)
>
>    el = Ellipse((0,0), 10, 20, facecolor='r', alpha=0.5)
>
>    fig = figure()
>    ax = fig.add_subplot(111, aspect='equal')
>    ax.add_artist(el)
>    el.set_clip_box(ax.bbox)
>    ax.annotate('the top',
>                xy=(nx.pi/2., 10.),      # theta, radius
>                xytext=(nx.pi/3, 20.),   # theta, radius
>                xycoords='polar',
>                textcoords='polar',
>                arrowprops=dict(facecolor='black', shrink=0.05),
>                horizontalalignment='left',
>                verticalalignment='bottom',
>                )
>
>    ax.set_xlim(-20, 20)
>    ax.set_ylim(-20, 20)
>    #fig.savefig('annotation_ellipse')
>
>
>
> fig.savefig('annotation_demo.png')
> show()
 
Hi John,

Thank you very much for your note !
How can I control the number of digits in the output (image.png) ?
I would like to have four digits for the real part and four digits for
the imaginary part.
And how can I suppress small numbers ?

        plot([data[-1].real],[data[-1].imag],'k+')
       
annotate(str(data[-1]),xy=(data[-1].real,data[-1].imag),xycoords='data')

Nils